1. Field of the Invention
The present invention relates to a technique for decoding encoded movie image data and, more specifically, compensation for excess or deficiency of filtering.
2. Description of the Related Art
There are techniques for encoding (compressing) and decoding (decompressing) movie image data to efficiently store and transmit digital images having an enormous amount of data. In Japan, as well as other countries, various encoding systems, including the motion picture experts group phase 2 (MPEG-2) used in digital terrestrial television broadcasting and H.264 standardized by the international telecommunication union telecommunication standardization sector (ITU-T), are used.
In particular, in encoding systems, such as H.264 and the audio video coding standard (AVS), post-filtering (deblocking filtering) is performed in order to eliminate block distortion caused by, for example, quantization executed during encoding of an image. Deblocking filtering (also known as smoothing filtering) is filtering for smoothing pixel values of pixels at a boundary of blocks into which a frame is divided, in accordance with a filter strength set in encoding.
For example, Japanese Patent Laid-Open No. 2008-048181 describes a technique for reducing block distortion by changing pixel values across an edge of blocks into which movie image data is divided through horizontal and vertical deblocking filtering.
However, depending on the filter strength of a smoothing filter set at the encoding side, an excessive removal of a high-frequency component of an image by filtering may result in image blurring, or filtering may be unable to offer sufficient advantages.
One such example case is the case where a user sets the filter strength for use in deblocking filtering higher than necessary in obtaining a movie image although there is no need of deblocking filtering (block distortion does not occur). In this case, if deblocking filtering is performed in accordance with the filter strength set in obtaining an image, a high-frequency component of the image would be excessively removed, resulting in the occurrence of image blurring.